In a CCD solid state imaging device or a MOS-type solid state imaging device, an efficiency of light collection to photosensitive units is increased by forming micro lenses for the purpose of improving sensitivity by increasing the amount of incident light to pixels. In this case, the micro lenses corresponding to the pixels are arranged in, e.g., a matrix pattern. In order to improve the sensitivity of a CCD or a CMOS sensor, it is required to enlarge an area of the micro lenses so that a large amount of light can be collected on a condensing point. To that end, a distance between adjacent micro lenses needs to be reduced. To be specific, there arises a need to reduce or remove a distance D1 between micro lenses 100 arranged lengthwise or transversely as shown in FIG. 16 and a distance D2 between micro lenses 100 positioned diagonally to each other.
Such micro lenses 100 have different high transmission wavelength areas or different condensing areas depending on their materials. Therefore, it is preferable to provide flexibility in choosing an organic material or an inorganic material, e.g., a silicon nitride film, a silicon oxide film or the like as a lens material depending on its usage. In order to form the micro lenses 100, there is used a semiconductor wafer W (hereinafter, referred to as “wafer”) in which an underlayer 101 having photosensitive units and a conductive film, a lens material layer 102 and a mask layer 103 formed of a resist film are laminated in that order from the bottom, as depicted in FIG. 17A, for example. As can be seen from FIG. 17A, the mask layer 103 is formed to have lens shapes. By etching the mask layer 103 and the lens material layer 102 by using a plasma of a processing gas, the lens shapes of the mask layer 103 are transcribed to the lens material layer 102, thereby forming the micro lenses 100.
The mask layer 103 that has been formed to have the lens shapes by a photolithographic process is softened by a heat treatment performed after the exposure process. Therefore, if the lenses are positioned close to each other, the lenses that have been softened adhere to each other due to a surface tension, which results in deformed lens shapes. Therefore, the lenses in the mask layer 103 are separated from each other by a distance d1 of, e.g., about 0.2 μm to about 0.5 μm, so as not to adhere to each other, and separated diagonally to each other by a distance d2 of, e.g., about 1 μm. Hence, the micro lenses 100 transcribed to the lens material layer 102 are also separated from each other by the distances corresponding to D1 and D2.
However, when the lens material layer 102 is made of an inorganic material, the distances D1 and D2 between the micro lenses 100 transcribed to the lens material layer 102 become greater than the distances d1 and d2 between the lens shapes in the mask layer (hereinafter, referred to as “initial distances d1 and d2”), as shown in FIGS. 17A and 17B in which the distances D1 and d1 are depicted representatively.
To that end, Japanese Patent Laid-open Application No. 2005-101232 has proposed a technique for reducing a distance between lenses in a method for forming micro lenses by using, e.g., a silicon nitride film. In this technique, a mask layer and a lens material layer formed of an Si3N4 film are etched by using SF6 gas and CHF3 gas as processing gases, while controlling a flow rate ratio of those gases. Accordingly, deposits are deposited on sidewalls of lenses formed in the mask layer to reduce a distance between the lenses in the mask layer, and the shapes of the lenses are transcribed to the lens material layer, so that a distance between micro lenses of the lens material layer is also reduced.
However, the present inventors have found that the above-described technique is not sufficient to reduce the distances D1 and D2, which hinders the improvement in sensitivity of a solid state imaging device using micro lenses made of an inorganic material. Consequently, it is difficult to obtain sufficient flexibility in choosing a material of micro lenses depending on its usage between an organic material and an inorganic material.